Compressed air supply systems are used in vehicles, especially commercial vehicles, to supply compressed air consumers in the vehicle with compressed air. Such compressed air consumers are service brakes, an air suspension unit, pneumatic transmission shift units or the like, for example. The air drawn in from the environment by the compressor generally contains moisture and impurities. Modern compressed air supply systems therefore comprise a device, referred to here in general as an air dryer, in which the compressed air made available by the compressor is freed from moisture by air drying means and from further condensates and particles by filters and separators in order to prevent damage to or functional impairment of the compressed air consumers.
Typical air dryers employ a desiccant which is regenerated after a certain time in operation by means of system air stored in a regeneration reservoir, passed via the air dryer and vented via an associated vent valve. The regeneration mode generally takes place when the operating pressure has reached a predetermined value, i.e. the compressed air system has been filled. The compressor is then switched over from a “delivery” operating state into a “standby” operating state by means of a governor, which is controlled by the service pressure and is generally pneumatic, preventing the system pressure from rising any further. On the other hand, the air dryer is switched over from a “delivery” operating state to a “regeneration” operating state, wherein the regeneration air taken from the regeneration reservoir generally flows through the air dryer counter to the delivery direction and is vented into the environment via the vent valve. The fixed cycle of “delivery” and “regeneration” predetermined by the system pressure alone does not take account of differences in the level of compressed air consumption and hence differences in the level of regeneration demand. In phases with a very high air consumption, it can happen that the available quantity of air is not sufficient in the case of normal system behavior (delivery up to a cut-off pressure, then one-time regeneration) to ensure sufficient drying, and therefore there is great variation in the quality of the available compressed air, not only from vehicle to vehicle but also during the daily operation of a vehicle.
DE 10 2010 024 476 A1 shows and describes a compressed air control method and a compressed air control device having a pneumatically actuatable governor of the kind already described above. To enable the compressor to be controlled even when the system pressure is not yet sufficient to switch the governor, an additional supply control valve is provided in a compressed air line bypassing the governor, by means of which valve the system pressure inlet can be connected pneumatically to the compressor control outlet in order to switch the operating state of the compressor, irrespective of the system pressure. It is thereby possible to supply the compressor control line with the system pressure, even when the system pressure is not yet sufficient to switch over the venting control valve. In this way, the compressor can be switched off or switched to a “standby” operating state without simultaneously triggering a regeneration process or purging process in the air dryer. The purpose of this measure is to enable the compressor to be switched off when, for example, the entire engine power is required to propel the motor vehicle. As before, control of the air dryer remains dependent on the switching state of the governor, which is controlled pneumatically by means of the system pressure, and therefore more efficient control of the air dryer is not possible.
The use of an electrically controlled governor to eliminate the rigid limits at which the governor switches over is also already known from U.S. Pat. No. 6,036,449. An electronic control device can supply the control line of the vent valve and the control line of the compressor with the current system pressure or vent these two control lines, irrespective of the respective system pressure. In this way, more flexible control of the regeneration of the air dryer is made possible. The purpose of this measure is especially to prevent the cylinder head temperatures of the compressor becoming too high and instead to keep them within predetermined limits so as to prolong the running time of the compressor. However, deliberately efficient control, especially of the air dryer, is not envisaged.